gpio: pca953x (i2c) handles max7310 too

The pca953x driver can handle another 8-bit I/O expander, the max7310.
This patch adds that chip to the list of supported IDs in that driver, and
expands the Kconfig helptext accordingly.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>